Light emitting diodes (LEDs) generate visible or non-visible light in a specific region of the electromagnetic spectrum. The light output from an LED may be, for example, light in the blue, red, green, ultra-violet (UV), and/or near-UV spectral regions, depending on the material composition of the LED. When it is desired to construct an LED light source that produces light of a color different from the output color of the LED, it is known to convert the light output from the LED having a first wavelength or wavelength range (the “primary light” or “excitation light”) to light having a second wavelength or wavelength range (the “secondary light” or “emission light”) using photoluminescence.
Photoluminescence generally involves absorbing higher energy primary light with a wavelength-conversion material such as a phosphor or mixture of phosphors. Absorption of the primary light can excite the wavelength-conversion material to a higher energy state. When the wavelength-conversion material returns to a lower energy state, it emits secondary light, generally of a different wavelength/wavelength range than the primary light. The wavelength/wavelength range of the secondary light depends on the composition of wavelength-conversion material used. As such, secondary light of a desired wavelength/wavelength range may be attained by proper selection of wavelength-conversion material. This process may be understood as “wavelength down conversion,” and an LED combined with a wavelength-conversion structure that includes wavelength-conversion material, such as phosphor, to produce secondary light, may be described as a “phosphor-converted LED” (pc-LED) or “wavelength-converted LED.”
The wavelength-conversion material may be formed into solid monolithic ceramic piece by an appropriate method such as pressing and sintering the powdered material. The ceramic wavelength converter, typically in the form of a plate, may then be attached directly to the LED die to achieve a chip-level-conversion (CLC) of the light emitted by the LED or it may be placed at some distance from the light emitting surface of the LED in order to achieve a remote-conversion arrangement. Transparent ceramic wavelength converters have the potential to generate the highest level of conversion efficiency for these applications because they do not suffer losses from backscattering converted light to the LED source where it can be absorbed. However, as ceramic wavelength converters are made with a higher degree of transparency, total internal reflection (TIR) becomes a limiting factor, restricting the amount of light that can be extracted from the converter and thereby limiting the efficacy of the light source.